I have a 120 back for an RB67. This one has a winding problem too, but it's easily dealt with by pressing on a certain spot when closing it, to be sure a tiny lever behind the edge is engaged. Once I figured this out, it has never failed me. I'd like to sell it with the body, which has no front cap, or with the body and 90mm lens, which also has no caps. (They were burgled from me some time back, and recovered this way.) If you'd like them in spite of these glitches, make me an offer; I'm not likely to refuse anything reasonable.

What problems? it doesnt wind around to 1? You can fix most problems. There is a silver lever inside the back that gets pressed in when closed with film, you could try attaching something to it so it gets pressed in more.

I have a couple of motorized Mamiya RB67-Pro-S 120/220 backs (used but like new and still boxed), but I'm not 100% sure about compatibility with your camera. They actually was spare backs for a film-recorder machine based on that Mamiya camera. Maybe some expert can advise about them...

Here's some pics:

This was one, the other one is exactly the same. Let me know if you are interested. Bye